This invention relates to a dental treatment device and, more particularly, to a dental treatment device for forming a screw hole for embedment of an implant material.
The conventional practice for stable maintenance of a denture has been to bury an implant material in the jaw bone below the soft tissue of a missing tooth and to set the denture thereon. The implant material is previously molded to have a threaded surface via which the material is screwed into a screw hole in the treated site formed by a tool bur of a dental treatment device. On the other hand, if a countertorque during formation of the screw hole exceeds an allowable value, an excess load is applied to the tool bur, thus giving rise to collapsed screw crests or errors in screw depth to thereby produce hindrance in the mounting of the implant material and occasionally the fracture of the dental treatment device. In order to prevent these drawbacks, there is known a dental treatment device in which a clutch unit is provided between an external electric motor and a speed-reducing unit for decelerating the rotation of the motor for affording a high torque necessary to form the screw hole. The clutch unit includes a motor side clutch plate and a tool side clutch plate in which a pair of indented surfaces of the clutch plates are axially engaged with each other by a spring. The dental treatment device is so designed that, when the tool bur is over-loaded, the indented surfaces are disengaged from each other against the bias of the spring and the motor side clutch plate is rotated with a click movement relative to the tool side clutch plate for reducing the torque of the tool bur.
With the conventional dental treatment device, in which the clutch unit is provided between the motor and the speed-reducing unit, those members which are disposed between the tool bur and the speed-reducing unit or between the speed-reducing unit and the motor side clutch plate, and which are relatively fragile against overload, such as a gear or a pin member for securing the gear to a rotary shaft, tend to be fractured during the time the countertorque is transmitted from the tool bur to the clutch unit. On the other hand, since the countertorque reached at the speed-reducing unit is increased in inverse proportion to a reciprocal of the deceleration ratio of the speed-reducing unit, the spring force of the spring responsible for adjustment of the disengagement of the indented surfaces from each other is difficult to set to a proper value.